In this age of increased air pollution, the removal of chemicals from the air we breathe is a concern of everyone. In addition, in the fabrication of electronic materials and of devices such as semiconductors, there is a requirement for uncontaminated air of high quality. To filter contaminants from the air, gas phase filtration is commonly employed, typically using activated carbon manufactured in various ways. One approach uses a carbon/adhesive slurry to glue the carbon to the substrate. The adhesive decreases carbon performance by forming a film on its surface. In another approach, an organic-based web is carbonized by heating, followed by carbon activation. Filters produced by such an approach is expensive and has relatively low adsorption capacity. In yet another approach, a slurry of carbon powders and fibers is formed into sheets by a process analogous to a wet papermaking process. This material has a medium-to-high cost, and has an undesirable high pressure drop. Moreover, chemically-impregnated carbon particles, used for the chemisorption of lower molecular weight materials, cannot be efficiently used in conjunction with an aqueous process, as the aqueous nature of the process either washes away the chemical used to impregnate the carbon, or reacts undesirably with the impregnating or active chemical groups thereby rendering it inoperative. In general, however, filter materials that do not incorporate chemically-active groups perform far less effectively for some key low-molecular-weight components, such as ammonia, in comparison to filter materials that include chemically-active groups.
Such filters have been accepted in the industry, and they are presumably considered to perform adequately for their intended purpose. However, they are not without their shortcomings. In particular, none of these aforementioned prior art approaches fully achieve the desired properties that provide a clean, cost effective, high efficiency, low pressure drop, adsorptive composite.
The present invention provides a filter which overcomes these shortcomings. In particular, in one aspect of the invention, a fluid-permeable filter includes a conduit through which fluid, particularly gas, can flow. Within the conduit is chemisorptive media that includes a copolymer having an acidic functional group for chemically adsorbing a base contaminant in a fluid passing through the conduit. Also within the conduit is physisorptive media for physically adsorbing a condensable contaminant from a fluid passing through the conduit. The chemisorptive media and physisorptive media are in separate filter elements in a preferred embodiment, though the two media types can alternatively be intermixed to form a single, undivided filter body.
Preferably, the filter is a clean, cost-effective, high-efficiency, low-pressure-drop, gas phase filter comprising a high-surface-area, highly-acidic, chemically-acidic adsorbent in combination with untreated, or virgin, activated carbon. One embodiment of the invention employs a non-woven composite material having acidic functional groups that bind to airborne bases. The untreated, activated carbon adsorbs organic and inorganic condensable contaminants, typically those having a boiling point greater than 150xc2x0 C. The invention can be used in lithography systems that employ materials that are sensitive to impurities, such as molecular bases (e.g., ammonia and n-methyl pyrrolididnone), and organic and inorganic condensable contaminants (e.g., iodobenzenes and siloxanes), present in the air circulating through semiconductor wafer processing equipment. A large number of bases including ammonia, NMP, triethylamine pyridine, and others, can be maintained at concentrations below 2 ppb in a tool cluster filtered with the present invention. The acidic adsorbent can be formed, for example, by the dry application of an active, acidic adsorbent to a non-woven carrier material that is then heated and calendered with cover sheets.
The non-woven carrier materials can be polyester non-wovens, and the acidic adsorbent can include sulfonated divinyl benzene styrene copolymer. One embodiment employs carboxylic functional groups. The acidic groups have at least 1 milliequivalent/gram of copolymer acidity level or higher and preferably at least 4.0 milliequivalents/gram of copolymer or higher. The polymers used are porous, and can have a pore size in the range of 50-400 angstroms and a surface area of 20 m2/g or higher.
The dry processing of a non-woven polyester batting allows for even distribution of acidic, adsorbent particles throughout the depth of the polyester batting. This provides an increased bed depth at a very low pressure drop, which is highly desirable since a twofold increase in bed depth can increase the filter""s breakthrough time (time to failure) fourfold when using these thin fabric-based sulfonic beds.
Activated carbon is discussed in greater detail in U.S. Pat. No. 5,582,865, titled, xe2x80x9cNon-Woven Filter Composite.xe2x80x9d The entire contents of this patent are incorporated herein by reference. The filter can have two (or more) layers, one of activated carbon and one of sulfonated divinyl benzene styrene copolymer beads. Additionally, two or more materials can be mixed to provide a composite filter.
Thus, provided herein is a clean, cost-effective, high-efficiency, low-pressure-drop, adsorptive composite filter, and a method for forming said composite filter. The composite filter is particularly useful for the removal of base and organic and inorganic condensable contaminants (typically those with a boiling point greater than 150 degrees C) in an air stream. Particulates will also be removed if greater than the pore size of the filter. The filter can have a service life of several months with a pressure drop to reduce power consumption and minimize impact on the systems operation. For example, a high-pressure-drop filter can require a longer time for a lithography system to equilibrate the temperature and humidity after filter replacement. In comparison to chemically-treated, activated-carbon filters, the combination filters of this invention offer much higher adsorption performance due to the superior adsorption properties of untreated, activated carbon over chemically-treated, activated carbon. The use of untreated, activated carbon in accordance with methods described herein can provide superior breakthrough capacity for organic and inorganic condensable contaminants because the chemical treatment performed on the activated carbon to render it suitable for capturing molecular bases compromises its capacity for adsorbing organic and inorganic condensable contaminants, typically those with a boiling point greater than 150 degrees C.
In another embodiment, a synthetic carbon material, such as that described in U.S. Pat. No. 5,834,114, the contents of which are incorporated herein by reference in their entirety, can be coated with the acidic materials of the present invention to provide a porous acidic filter element in accordance with the invention. In yet another embodiment, the activated nutshell carbon media described in U.S. Pat. No. 6,033,573, the contents of which are incorporated by reference in their entirety, can be used alone or in combination with any of the other chemisorptive or physisorptive media described herein to remove contaminants from the air flowing through the conduit in the same manner as is taught in this specification.
A detection system and method of use for determining when the filter needs to be replaced by detecting base contaminants in air is described in U.S. patent application Ser. No. 09/232,199, entitled, xe2x80x9cDetection of Base Contaminants in Gas Samples,xe2x80x9d filed on Jan. 14, 1999, now U.S. Pat. No. 6,207,460, issued on Mar. 27, 2001, with Oleg Kishkovich, et al. as inventors. Also, U.S. patent application Ser. No. 08/795,949, entitled, xe2x80x9cSystem for Detecting Base Contaminants in Airxe2x80x9d, filed Feb. 28, 1997, now U.S. Pat. No. 6,096,267, issued on Aug. 1, 2000, with Oleg Kishkovich, et al. as inventors, and U.S. patent application Ser. No. 08/996,790, entitled, xe2x80x9cProtection of Semiconductor Fabrication and Similar Sensitive Processes,xe2x80x9d filed Dec. 23, 1997, now U.S. Pat. No. 6,296,806, issued on Oct. 2, 2001, with Oleg Kishkovich, et al. as inventors, can also be used with the present invention. These patent applications disclose the protection of a DUV lithography processes using chemically-amplified photoresists that are sensitive to amines in the air. These patent applications are incorporated in the present application in their entirety by reference.
One method of fabricating a filter element having a large surface area and the desired flow characteristics involves the use of a powdered material that is deposited in sequential layers one on top of the other. Following the deposit of each layer of powdered material, a binder material is delivered onto each layer of powdered material using a printing technique in accordance with a computer model of the three dimensional filter element being formed. Following the sequential application of all of the required powder layers and binder material to form the part in question, the unbound powder is appropriately removed, resulting in the formation of the desired three dimensional filter element. This technique provides for the fabrication of complex unitary or composite filter elements having high surface area that are formed with a very high degree of resolution.
In another apparatus, the physisorptive and chemisorptive filter media are positioned in a circulation loop for circulating air through a photolithography tool. The two media are respectively positioned at different locations such that the physisorptive media will be maintained at a temperature cooler than that at which the chemisorptive media is maintained.
The physisorptive media can be positioned upstream from the chemisorptive media (i.e., between the chemisorptive media and an outlet of the photolithography tool) and can be positioned proximate to the downstream side of a cooling coil in the air conditioning unit of the tool. Alternatively, the physisorptive media can be coupled with a separate cooling element, such as a source of chilled water. In either case, the air passing through the physisorptive media can be cooled and then, after exiting the physisorptive media, reheated to a fixed temperature and passed through the chemisorptive media before re-entering the photolithography tool. Temperature sensors can be used to monitor the temperature of the different media and also provide feedback signals to a controller for closed loop control of the system. The physisorptive filter element can also be contained in a rotating wheel with separate chambers for active adsorption, regeneration and conditioning. Advantages provided by some of these embodiments include enhanced removal of lower-molecular-weight condensable contaminants, reduction in the overall footprint of the system, reduction in operating pressure drop of the filtration component, and significantly increased time between change-out or service. Further, lower-molecular-weight organic contaminants may be removed more effectively with the temperature-swing beds described herein than is achievable with passive adsorption beds.
In another aspect of the invention, a filter unit include a multiplicity of filter elements. The filter elements are made of a chemisorptive media and a physisorptive media. The filter unit also includes a multiplicity of sampling ports within the filter unit for connecting to a monitor device which monitors the performance of the filter elements. The sampling ports are arranged in a manner with individual sampling ports located between adjacent filter elements. There can be sampling port located on an upstream side of the multiplicity of filter elements, and another sampling port located on a downstream side of the multiplicity of filter elements.
In some embodiments, the monitor device is an analytical device, such as, for example, a gas chromatograph mass selective detector, an ion mobility spectrometer, an acoustic wave detector, an atomic absorption detector, an inductance couple plasma detector, or a Fourier transform methods. Alternatively, the monitor device can be a concentrator which collects the sample drawn to the concentrator with a pump, or the concentrator is coupled to the sample port so that the contaminants accumulate in the concentrator by diffusion. Once the sample is collected in the concentrator, the concentrator is taken to a lab for evaluating the sample. The filter elements can be arranged in a set of stack which are arranged in a series, and in each stack, the filter elements are arranged in parallel.
In another aspect, a photolithography system includes an air handler for moving air through the system, and delivers unfiltered air to the filter unit, and a photolithography tool which receives filtered air from the filter unit. A particular advantage of this arrangement is that it is able to detect contaminants before the contaminants reach the lens of a photolithography tool.
In yet another aspect of the invention, a filter unit includes one or more filter elements. There can be a sampling port located between two filter elements. Additionally, or alternatively, there is a sampling port located on one side of a filter element, or there can be a second sampling port located on an opposite side of the filter element.
Related aspects of the invention include a method for filtering air through a filter unit and a method for circulating air through a photolithography tool.